1. Field of the Invention
The present invention relates to systems in which different modules accept different voltage levels, and more particularly to translation from one voltage level to another and to overvoltage protection of modules accepting lower voltage levels.
2. Description of Related Art
Many modern systems combine modules accepting different voltage levels, for example, modules powered by different power supply voltages. For example, in battery powered laptop, notebook and hand-held computers, some modules are powered by a 3.3 V voltage while others are powered by a 5.0 V voltage. The use of the lower power supply voltage such as 3.3 V reduces power consumption. However, modules such as disc drives are powered by 5.0 V because they perform better when powered by the higher voltage.
To translate a signal produced by a 3.3 V module to a level acceptable by a 5.0 V module, one can use a CMOS inverter powered by 5.0 V. The inverter powered by 5.0 V receives a signal from the 3.3 V module and provides a voltage level acceptable for the 5.0 V module. However, the 5.0 V CMOS inverter consumes much power when its input is at 3.3 V. There is a need therefore for a power efficient voltage translator.
Further, when the output driver of the 3.3 V module is disabled and the 5.0 V module drives a common bus with 5.0 V, a charge leakage path may form between the bus and the 3.3 V power supply. For example, suppose that the output driver of the 3.3 V module includes a PMOS pull-up transistor connected between the 3.3 V voltage and the bus. When the bus is driven by the 5.0 V module, the 3.3 V module drives the PMOS transistor gate to 3.3 V to turn the transistor off to disable the driver. However, when the bus voltage rises to 5.0 V, the PMOS transistor turns on providing a conductive channel between the bus and the 3.3 V power supply. Moreover, because the PMOS transistor backgate is typically held at 3.3 V, the drain/backgate diode turns on providing another conductive path between the bus and the 3.3 V power supply. The undesirable results include loading the bus and causing "bus contention", degrading the bus signals, and causing possible false signals on the bus.
Similar problems occur when a selected module is powered down while other modules are powered up, whether or not different power supply voltages are used in the system. When the modules powered down in order, for example, to save power or to replace the module, a leakage path can form between the bus and the module internal power supply.
Thus, it is desirable to provide output drivers which do not provide a leakage path between the bus and the module internal power supply.